Prolonged therapy is often required for diseases produced by sequestered microbes such as Mycobacterium tuberculosis and has been associated with the emergence of drug resistant mutants. Therapeutic drugs such as isonicotinic hydrazide, ethambutol and streptomycin and others are therefore used in combination in order to eliminate populations of resistant organisms or at least to delay their appearance. Certain antibiotics such as rifampin have been reserved primarily for the treatment of tuberculosis in this country. Viomycin and streptomycin show some cross resistance and both are more active on Mycobacterium than on Escherichia. Other compounds such as isonicotinic hydrazide and ethambutol are active only on mycobacteria. Analysis of the mode of action of several of these drugs on the myobacteria has been aided by the use of cell free systems employing ribosomes or DNA dependent RNA polymerase from the non pathogen, Mycobacterium smegmatis. Knowledge of the mechanisms of the antibacterial action of these compounds will aid the future development of other drugs or the chemical alteration of present ones since such information provides insight into the biosynthetic reactions of mycobacteria and may thus indicate useful macromolecular targets for drug action. This research project attempts to show the effect of ethambutol on translation and transcription in cell free extracts of drug sensitive and drug resistant Mycobacterium smegmatis. Ultimatey the selective specificity of ethambutol for mycobacteria may be explained.